I was honestly skeptical of another format beyond AVIF for images. I thought the next generation coded would be based on neural net encoding/decoding for lossy images (as we should be able to get "easy" 2x wins). However, I think progressive loading in JPEG XL is an amazing feature, and a significant update, and is perhaps more important that raw compression ability.

My understanding is also that you can also use the progressive nature to have 1 "high res" version, and potentially render different sizes for different connections, and do some interesting things. For example, I believe you could (in principal) do something like stop loading after 1s so long as a minimum size level was achieved) - to make things fast on all devices, but only the quality would vary. So if you are on a 4k monitor and gigabit download? You can get massive images. On a 3g phone? Get the basics.

It's a really great feature, and does seem like it would likely have the most real world impact on actual website perceived performance.